[dart:io] Remove DART_IO_DISABLED

runtime/bin/sync_socket_linux.cc

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-#if !defined(DART_IO_DISABLED)
-
 #include "platform/globals.h"
 #if defined(HOST_OS_LINUX)
 
 #include "bin/sync_socket.h"
 
 #include <errno.h>  // NOLINT
 
 #include "bin/fdutils.h"
 #include "bin/socket_base.h"
 #include "platform/signal_blocker.h"
@@ skipping 68 matching lines @@
 }
 
 void SynchronousSocket::Close(intptr_t fd) {
   return SocketBase::Close(fd);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_LINUX)
-
-#endif  // !defined(DART_IO_DISABLED)